Knowing this, the grass roots group behind a 20-mile pedestrian loop known as the Green ARTery invited Ryan Gravel to share his story Saturday at a dinner celebrating the trail.

Gravel was a college student studying abroad in the 1990s, whose daily walk to the market took him under an abandoned elevated railroad in Paris. He was surprised to watch it transform into the Promenade Plantée, an elevated park that later inspired another, the High Line in New York City.

Gravel's master's thesis in Architecture and City Planning from Georgia Tech is the vision behind the Atlanta BeltLine, a 22-mile loop that brings together a modern streetcar and green trails on an old railway corridor.

Urban trails are growing all over, he said, in different conditions and scales, some as short as a mile and others as long as a 52-mile greenbelt along the San Diego River.

"What they share is the ability to transform life for people who live there," Gravel said.

They are all driven by grass roots efforts.

In Tampa, the Green ARTery started two years ago as a combined dream of Lena Young-Green, a retired senior legislative assistant to former state Sen. James Hargett, who lives in Tampa Heights, and Myron Griffin, an interior plantscaper and antique dealer in Hampton Terrace.

Two years have passed and pieces of the trail are in place. Trail organizers and people who live in the 20 neighborhoods within the loop are celebrating Saturday with games, food, art and groups exploring the trail.

The multiuse trail follows the Hillsborough River from 40th Street to downtown. Then it links with the Riverwalk downtown, before curving around Palmetto Beach to McKay Bay and back up 40th Street.

At conception, many of these trails seem too ambitious.

"What started as a kernel of an idea turned into something we never could have imagined," Gravel said. "We're building a really broad constituent base. That's what's driving it."

The BeltLine has a 2-mile section of trail complete. The streetcar is still in the planning stage.

"In fact, we don't even own the whole loop," Gravel said. "But it has momentum. The public is invested."

Thousands of people came together on the trail this month for the BeltLine Lantern Parade.

Gravel recently took his son biking on the trail. When he got home, he looked at pictures he had taken and noticed a woman in the background carrying groceries.

It is changing the way people live, Gravel said.

Elisabeth Parker can be reached at eparker@tampabay.com or (813) 226-3431.

. If you go

Green ARTery celebration

Starts at 8 a.m. Saturday with a bike rodeo, games and raffles at Fair Oaks Community Center and Carrington Community Park, 5019 N 34th St.